ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
433 
balance showed to be several milligrams too heavy, has given the unlooked for 
l'esult that no corresponding quantity of chloride was present. Something more 
than a mere trace was indeed detected, but of amount probably not exceeding 
^ milligram The deposit from chlorate and acetate of April 2, and another which 
does not appear in the table as the current weighings were not taken successfully, 
in which the excess was about 7 milhgrms. were both treated in this way with similar 
results. The loss of weight on strong heating appears to exclude the supposition 
that though chlorate was present it escaped decomposition, and thus we seem almost 
driven to the conclusion that the redundant matter is principally acetate, although 
the proportion of acetate to chlorate in the solution is a small one. 
§ 22. We have had occasion to examine another point relating to the chemistry of 
electrolysis, of which the result may here be recorded. In our earlier experiments we 
used anodes containing an appreciable quantity of copper. The copper evidently 
tended to accumulate in the solution, becoming after a time apparent by its colour 
even when neutral; on addition of ammonia a distinct blue was struck. We were 
desirous of ascertaining whether under these circumstances there is danger of the 
deposits becoming contaminated. A distinctly blue solution was prepared, in which 
the proportion of copper to silver was considerable, and a deposit made. The texture 
was very much modified by the action of the copper, and the appearance was such 
that it was difficult to believe that the weight could be more than a small fraction of 
that of the simultaneous deposit from a pure silver solution. Some of the metal, 
which adhered very loosely, was lost in the washing, but the weights agreed to within 
a few milligrams. On dissolution in nitric acid and supersaturation with ammonia the 
solution showed no trace of colour, although about jotVoo °f copper can thus be 
detected. 
§ 23. In the absolute measurements the determination of the interval (never less 
than three-quarters of an hour) between the first passage of the current through the 
voltameters and its final cessation could readily be effected with sufficient accuracy 
(probably to touToo)> but a slight correction is called for in order to take account of 
the loss of time incurred at each operation of the reversing key, which controlled the 
direction of the current in the fixed coils (§ 8). To obtain the necessary data for this 
correction the main current was led through a few turns of wire surrounding a reflect¬ 
ing galvanometer. The resulting deflection is independent of the position of the key, 
but at the moment of reversal the current is interrupted, and the spot of light falls 
back towards zero. From a comparison of the amount of this falling back with that 
of the steady deflection, in conjunction with observations of the period of vibration, 
it is easy to deduce the time of interruption. It proved to be less than -y 0 - second, 
and was so nearly constant that after sufficient experience had been gained further 
observations were judged to be unnecessary. The connexions for this purpose are 
accordingly not shown in the diagram (Plate 17, fig. 1). 
§ 24. In order more fully to explain the procedure in taking a deposit it will be advisable 
MDCCCLXX XIV. 3 K 
